Rigid risers are important devices used in offshore engineering to transfer fluids in both directions between the seabed and the vicinity of the sea surface. In particular production risers are used to transport hydro-carbons from the seabed to fixed and/or floating platforms or vessels.
During the life of production risers the flow of hydrocarbons can be accidentally stopped by formation of hydrate plugs that can be difficult to clear—it may take several months to clear a hydrate plug. Considerable costs can be incurred during the efforts to restore the production that add to the losses caused by the interruptions in the production.
Intervention-Y units are occasionally installed on catenary production risers in order to facilitate coiled tubing and/or wireline operations. Coiled tubing and/or wirelines can be used to clear hydrate plugs for example by dissolving them with methanol. The Intervention-Y installations are generally regarded as optional and are rare. They are however missed when they become necessary and are not installed. The reasons for the above mentioned rare use are two-fold:                additional investment costs,        there is at present no easy, established way of utilizing the intervention-Ys.        
Steel catenary riser hang-offs tend to be located near the keel depths of floating platforms (approximately 100 ft to 300 ft below the sea surface), and Intervention Ys are difficult to access from the surface through the layers of the water column typically featuring the strongest wave and current actions. Adding intervention-Ys to existing risers later during the life of the field may be difficult and costly, even if feasible.
At this time two known approaches tend to be considered while adding a servicing riser between an Intervention Y unit and installations on the surface:                A provisional multi-span riser installation,        A provisional tensioned riser/jumper installation (for example approximately 200 kip top tension).        
Neither of the about provisional methods are straightforward. It may be difficult to provide provisional locations for supporting clamps. Also Steel Catenary Risers (SCRs) tend to be inclined around 10° to the vertical in the regions of their hang-offs and the change of direction between that tangent to the SCR pipe and the service flange of an Intervention Y and the vertical direction required for access to the servicing equipment above the sea surface. Often the proximity of essentially vertical platform column, truss structure, pontoon or side wall often come in the way. The above listed types of difficulties often arise when either of the above listed provisional riser installations are considered. For a provisional tensioned riser/jumper an additional difficulty may be on the side of the structural strength limitations of an Intervention Y already installed.
The mini-riser configurations suggested herein are designed to overcome the above highlighted difficulties.
Intervention Ys considered herein are those suitable to be installed on rigid catenary risers, or metallic catenary risers, SCRs, Lazy Wave SCRs or Steel Lazy Wave Risers (SLWRs), or a titanium catenary risers, or a Chinese lantern risers, or a bottom weighed risers, or a fiber reinforced plastic catenary risers, or a fiber reinforced plastic lazy wave risers.
Rigid catenary risers, or metallic catenary risers, SCRs, Lazy Wave SCRs or SLWRs, or a titanium catenary risers, or a Chinese lantern risers, or a bottom weighed risers, or a fiber reinforced plastic catenary risers, or a fiber reinforced plastic lazy wave risers are essentially variations of SCRs and without a use of Intervention Ys like those described herein coiled tubing or/and wireline operations would not be possible in those risers. It is known to anybody skilled in the art that all the above highlighted risers generally connect to subsea pipelines provided on their other ends with Pipeline End Manifolds (PLEMs), Pipeline End Terminations (PLETs) or skids used to connect subsea pipelines to continuing subsea pipelines. Those in turn are usually connected to subsea wells using jumpers.
It is also known to anybody skilled in the art that subsea wells can be accessed from the surface using tensioned or freestanding drilling, completion or workover risers that allow direct drilling, coiled tubing, wireline operations and tool access essentially vertically from the surface using standard intervention equipment. However, accessing subsea pipelines from the side of subsea wells is even more difficult than it is from the tops of rigid catenary risers, or a metallic catenary risers, SCRs, Lazy Wave SCRs or SLWRs, or a titanium catenary risers, or a Chinese lantern risers, or a bottom weighed risers, or a fiber reinforced plastic catenary risers, or a fiber reinforced plastic lazy wave risers, and that provides an option for utilizing mini-risers according to this invention.
It immediately follows for anybody skilled in the art that intervention operations using top tensioned or freestanding risers in subsea wells or in surface wells are not of a concern with regard to the use of this invention.